1. Field of the Invention
Embodiments of the present invention relate to heat exchangers and, in particular, to hybrid heat exchangers and cores used to build such heat exchangers.
2. Description of the Related Art
Heat exchangers are engineering devices that have found widespread utility in applications such as refrigeration, air conditioning, power production, and chemical processing. Heat exchangers are often used in machines and industrial processes, wherein the core of the heat exchanger facilitates transfer of heat from one fluid to another in order to perform functions such as cooling or heat recovery.
In one embodiment, a heat exchanger core includes a series of parting sheets which are stacked upon each other. Each parting sheet is separated from its neighbors to form a fluid flow passageway. In operation of the heat exchanger, hot and cold fluids are passed through the core in adjacent passageways and heat from the hot fluid is transferred to the cold fluid by conduction of heat through the parting sheet. Bridging elements are often interconnected to the two plates within the fluid flow passageways as well. These elements are in thermal communication with the plates and increase the surface area of the heat exchanger in contact with the two fluids. In this manner, the bridging elements also transfer heat by conduction through the bridging element to the parting sheet and subsequently to the cold fluid or to the cold fluid directly. An example of a heat exchanger is the automobile radiator, where a first fluid, hot coolant, is contained within the body of the radiator and a second fluid, ambient air, is blown past the surface of the radiator. The radiator body functions as a parting sheet, receiving heat from the hot coolant and transferring it to the relatively cool, flowing air.
To perform this heat transfer function, the core is subject to several performance requirements. These include heat transfer between the fluids, mechanical strength to support internal pressures from fluid flow and thermal stresses induced at operating temperatures, and substantially little leakage of the fluids. Aerospace and military industry applications further demand lower weight and more efficient heat transfer. For example, little to no leakage is allowed in land-based heat exchangers, while substantially all space bound heat exchangers operated under vacuum do not allow any leakage.
In traditional high temperature heat exchanger design, all-metal fabrications have been used to meet these demands. Metal fabrications, however, possess inherent limitations which are problematic for the more demanding aerospace and military applications. For example, while aluminum is light weight and possesses excellent thermal conductivity, it is limited to applications below approximately 500° F. because of softening above this temperature. Similarly, while Ni- or Fe-based alloys are often utilized for higher temperature applications, in the range of 700-1100° F., these alloys are heavy and exhibit low thermal conductivity, resulting in high weight and low thermal effectiveness. Furthermore, metals possess a relatively high coefficient of thermal expansion (CTE), resulting in high thermal stresses between different members of the heat exchanger which are typically operated at different temperatures. Additionally, metals are subject to corrosion in aggressive environments, which limits the durability and lifetime of all-metal heat exchangers
From the foregoing, it is apparent that there is a need for an improved heat exchanger. In particular, there is a need for a high temperature heat exchanger with improved heat transfer and leak tight which further possesses reduced weight.